Magnet control



Aug. 4, 1953 w. J. HUDSON 2,648,033

' MAGNETA CONTROL Filed Sept. 24, 1951 2 Sheets-Sheet 1 :L I Z 7 Patented ug. 4, 195.3

MAGNET CONTROL William John Hudson, Milwaukee, Wis., assignor to Allen-Bradley Company, Milwaukee, Wis., a corporation of Wisconsin Application September 24, 1951, Serial No. 248,014

9 Claims.

This invention relates to control circuits for electro-magnets such as lifting magnets and it more specifically resides in a control circuit having switch means adapted to connect a magnet to be controlled toa power source for energization and having a bridge network incorporating the magnet within an arm thereof, the bridge net work being connected to the power source during periods of deenergization of the magnet through switch means operative in response to conditions of unbalance within the bridge due to voltages of self-induction within the magnet that occur upon opening the switch means for energization and the bridge network upon co-nnection to the power source impresses upon the magnet a voltage the polarity of which is reversed from that impressed during energization thereof.

Upon opening an inductive circuit, in which direct current is flowing, voltages of self-induct- 'ance due to the sudden collapse of current within the inductive elements of the circuit may greatly exceed the impressed voltage values. Further if such inductive elements comprise windings related to magnetic material, the retentivity characteristics thereof may cause a substantial residual magnetization to prevail after the interruption of energizing current. Such phenomena are undesirable in the use of inductive apparatus such as lifting magnets and control circuits fo-r use with such inductive apparatus are employed to limit peak voltage values of self-inductance and to minimize residual magnetism.

' Lifting magnets, when in operation, are seldom at rest other than at initial moments of lift and attraction of a load, and for effective operation it is desirable that release of all the load be effected promptly at the will of the operator. Otherwise, deposit of the load may not be pin pointed as desired and a scattering thereof will result. To gain the desired operation the energizing or magnetizing current should be `decreased rapidly and the retentivity of both the magnet core of the magnet and of the load must be compensated for by a demagnetization accomplished' by a reversal of current flow that reduces residual magnetism to a value inelective for support of the load. It has been the practice heretofore to provide in the control circuit a switching arrangement such as to join the device to be controlled with a discharge path to conduct current caused by voltages of self-inductance thus aiming to limit peak voltage.` rIt has further, heretofore, been the practice to join the v,deviceto the power source for a short duration 2 with the polarity reversed from that of the initial energization or magnetization with the aim of causing the desired demagnetization necessary for a clean break of the load from the magnet.

The aims above mentioned have been accomplished with some measure of success by manual switching in which the operator is called upon to judge the time during which the demagnetizing currents of reverse polarity are to be applied to the inductive device. However, if the residual ux in the device and load is not adequately diminished the load may not be released in its entirety and the amounts released may drop spasmedically. On the other hand, a prolonged application of demagnetizing current may build up a reverse magnetization flux of value suicient to retain some part or all of the load, thereby hindering efiective operation. Since the degree of magnetization obtained is partially dependent both upon magnetic properties of the load and the parti-cular magnet, even an experienced operator will have considerable difficulty.

The magnet being inductive, the rate of demagnetization of residual due to an adequate reverse current is dependent upon the. magnitude of an adequate reverse voltage and the duration of its application. Thus, a large reverse polarity voltage `applied -for a short duration will have the same effect as a smaller voltage applied and maintained for a longer interval. Consequently a control circuit for lifting magnets and the like should, for optimum performance, provide for a rapidly building reverse-current which is terminated at a precisely determined value. It is an object of this invention to provide a control which, while retaining the advantages of protection against the adverse effects of peak voltages of self induction, may be adjusted to provide rapid build up of reverse current and preciseand prompt termination of the same at the value desired.

It is another object of this invention to provide a magnet control circuit having the foregoing advantages which will operate to produce the desired results without dependence upon the skill of the operator.

It is a further object of this invention to provide a control circuit having the foregoing advantages in which the magnet to be controlled is connected with a discharge circuit separate from the power source.

These and other objects and advantages of this invention will appear in the description to follow. In the description reference is made to the accompanying drawings in which there is shown by way of illustration and not of limitation certain forms in which this invention may be embodied.

In the drawings:

Fig. 1 is a circuit diagram of a control circuit for a lifting magnet in which one form of the circuit of this invention may be embodied,

Fig. 2 is a schematic circuit diagram of the circuit shown in Fig. 1 the component elements being arranged in across the line form for purposes of simplication,

Fig. 3 is a circuit diagram of another form of a magnet control circuit of this invention, VV and Fig. 4 is a circuit diagram of still another form of a magnet control circuit of this invention.

Referring now to the drawings, there is shown in Fig. 1 a lifting magnet I having a core of magnetic material in which is` embedded an inductive winding Y2v. The magnet I is of Well known form commonly employed in connection with hoists and cranes for the purpose of handling magnetic materials. The winding 2 is connected by leads 3 and 4 to terminals 5 and 6 of a controller unit 1, outlined in Fig. 1 by dotted lines. current power source, represented by the leads 8 and 9, is connected through a main switch I0 by input leads I I and I2 to the power input terminals I3 and I 4 of the controller unit 1.

Lead I5 connected at one end of input terminal I3 branches at its other end inte lead I6 joined to a normally open contact I1 of a lift contacter generally designated by the numeral I8 and into lead I9 joined to a normally open contact of a drop contacter generally designated by the numeral 2i. From the contact I1 a lead 22 extends to one side of contact 23 of the centactor I8 and from the opposite side of the contact 23 a lead 24 runs to a peint of juncture with a lead 25 which joins in turn with terminal 5, connected through the lead 3 to the magnet coil 2. From the contact 20 a lead 26 extends to one side of contact 21 of the contacter 2l and from the opposite side of the Contact 21 a lead 28 runs te one of the tapped terminals of a resistor 29.

A lead 38 is connected at one end to the input terminal I4 and branches at its other end into a lead 3l connected to a normally open contact 32 of contacter I3 and into a lead 33 that is connected to a normally closed contact 34 of the contacter 2l. Joined to the lead 33 is a short lead 35 that is connected at its opposite end te a normally open contact 36 of the contacter -2I. Y From the contact 32 a lead 31 extends to and joins with a lead 38 which is connected at one end to an end terminal of a potentiometer 39 and at the opposite end to a lead 48 which in turn is connected between the fixed terminal of the resister 29 and one end of a resistor 4I. From the opposite end of resistor 4I a lead 42 extends to a normally closed contact 43 ef the contacter I8 and a lead 44 joined to the opposite side of the contact 43 leads te 'one side of a resistor 45. From the opposite side of the resister 45 a lead 46 extends to one side of a resistor 41 and a lead 48 which is also connected to the resistor 45 is joined with the lead 24. The resistors 45 and 4I form part of a path in shunt with the winding 2 which is completed by a lead 49 extending between the terminali? and the lead 38.

Joined to the right hand terminal of the resistor 41 is a lead 50 which runs to and joins with one side of the contact 36 of the contacter 2I. Branching from the lead 50 is a lead 5I which is connected to the upper terminal of the po- A direct tentiometer 39. From the mid terminal of the potentiometer 39 connection is extended through a lead 52 with the operating coil 53 of the contacter 2I and thence through a lead 54 and a rectier 55 with the lead 24.

A push button control station 56 indicated by a breken line enclosure in Fig. 1 comprises a pilot light 51, a normally open manually closeable lift contact button 58 and a normally closed manually opened drop contact button 59. From the right hand side ef the light 51 a lead 50 extends to the power input terminal I4. The left hand side ofthe light 51 is joined by a lead BI to the left hand side of the lift button 58 and a lead 62 also connected to the left hand side of the lift contact button 58 is joined to a terminal 83 of the controller panel 1. The right hand side of the lift contact button 58 is connected te a terminal 64 on the panel 1 through a lead 65 and additionally joined through a lead 66 to one side of the drop contact button 59. The opposite side of the drop contact button 53 is joined te a terminal 61 of the panel1 through a lead 68.

Terminal 51 of controller 1 is joined through lead with lead I5 and thence with the power source. 'Ihe terminal 63 is joined to one side of a normally open contact 69 of the contacter I8 through a lead 10 and the terminal 64 is joined te the other side of the contact 69 through a lead 1I. From the lead 10 a lead 12 extends to the lower end of a resistor 13 yand a lead 14 leads from the upper end of the resistor 13 to one side of a normally closed contact 15 of the contacter I8. 'Ihe other side of the contact 15 is joined to the lead "I2 through a short lead 16. 'Io cemplete the controller circuit the operating coil 11 of the contacter I8 is joined through lead 18 te the lead 14 and through lea'd 19 to the contact 34 of the contacter 2 I.

In order to facilitate description of the mode of operation of the practical embodiment of the invention shown in Fig. 1, the circuit thereof has been redrawn in Fig. 2 in simplified ferm. In simplifying, certain leads of the practical embodiment have been consolidated. For this reason reference numerals have been omitted from the leads. Contacter coils and contacts shown in Fig. 2 are designated by the general 'designating numerals employed in Fig. 1 for the contactors as a whole and accompanying each is the specific numeral, enclosed in parentheses, that is employed in Fig. 1 for the designation of the particular coil or contact.

In operation the apparatus shown in Figs. l and 2 is rst activated by closing line switch I0 placing the direct current power supply across the contacts of the normally open lift push button 58 of the control station 56. To initiate the magnetization of the lifting magnet I the operator depresses the normally open lift push button -58 of the controller station 56. A 'circuit through power line 8, switch I0, leads I I, I5, and 8D, normally closed push button 59, lead 66, the depressed button 58, leads 62, 10 and 16, normally closed-contact 15 of the contacter 'I 8, `leads 14 and 18, winding 11 of the contacter I8, lead 19, 'normally closed contact 34 of the contacter 2|, leads 33, 30 and I2, switch Il! and power line 9 is thus completed to energize the winding 11 and pick-up the contacter I8. The contacts I1, 23 and 32 close to place the winding 2 of the magnet I across the direct 4current power source and contact 43 opens to remove the resistors 4I andv 45 from closed circuit with the magnet Winding 2.

The energizing circuit for the magnet I, thus established extends from the lead 8 and the switch I0 through leads II, I5 and I6, contacts I1 and 23, leads 24, 25 and 3, the magnet winding 2, the leads 4, 49, 38 and 31, the contact`32 and the leads 3|, 30 and I2 to the switch I0 and the power line 9. The polarity applied is such that the rectifier 55 blocks current from the winding 53 of the contactor 2| and a large total resistance Value is provided in resistor 41 and mainly in potentiometer 39 so as to allow only a small current drain therethrough.

Upon closure of the contactor I8 contact 69 closes to establish a maintaining circuit after release of the button 58, while the contact 15 of the contactor I8 opens to place the resistor 13 in circuit with the coil 11 to limit the current therethrough after pick-up of the contactor I8. Closure ofv either or both of contacts 58 and 69 causes lamp 51 to be lit, to indicate the lift condition.

For the purpose of demagnetizing and controlling magnetism and the rate of decay of iiux and consequently peak induced voltages a discharge circuit is provided and provision is made for application of reverse` current to the winding 2 under the automatic control of a four armed four cornered bridge circuit of which the magnet coil 2 forms part of one arm.

By referring to Fig. 2 the makeup of the discharge and bridge circuits may be more clearly discerned. The discharge circuit comprises the resistors 45 and 4|, the contact 43 and associated leads. The magnet winding 2 forms, in conjunction with the discharge circuit, one arm of the bridge. A second arm for the bridge circuit is formed by the resistor 41 and the remaining two arms by the two sections of the potentiometer 39. The resistances of the latter two arms may be relatively varied by movement of the slider of potentiometer 39. The responsive circuit of the bridge, which joins the corner of the bridge formed by the slider of the potentiometer 39 to the diagonally opposite corner at the junction of resistor 41 and winding 2, comprises the rectifier 55, the coil 53 of the` contactor 2| and associated leads.

The discharge and bridge circuits are brought into play upon depressing the normally closed push button contact 59 of the control station 56 to open the energizing circuit of the coil 11 rof the contactor I8. Upon relaxation of the contactor I8 the contact 43 thereof which isarranged to close a slight interval before the opening of either of the contacts I1, 23 and 32 joins the discharge resistors 4| and 45 in shunt relation with the magnet winding 2 through the circuit comprising leads 3, 25, 24 and 48, the resistor 45, lead 44, the contact 43, the lead `42, the resistor 4I and the leads 49, 38, 49and 4. Highr voltages of self-inductance of the winding 2 are thus isolated from the voltage source and a discharge path comprising theA resistors 4| and 45 is provided to limit the peak voltage values appearing across the winding 2.

The self-induced voltages occurring on discharge are of reversed polarity as to which the rectier 55 is conducting. rThe self-induced Vvoltage thus causes current to ilow through the responsve circuit element of the bridge.. The current path involved leads fromfthe magnet winding 2through leads4 4, 49 and 38,.partof the po- .tentiometer393to theslider thereof,- and thence through lead 52, coil 53 of contactor 2|, 1ead'54, rectifier 55, and leads 24, 25 and 3 to the return terminal of the winding 2. Contactor 2| is thus activated and the closure of contacts 20, 21 and 36 now occurs to connect the bridge circuit to the power source at the two diagonally opposite corners of the ends of potentiometer 39. While this condition prevails contact 34 is maintained open to ensure non-operation of the contactor I8 in case of an attempted operation of the push button 58.

With the power source thus applied to the bridge circuit current is urged to flow through the bridge arms comprising resistor 41 and magnet winding 2 in a direction reverse from that pursued in the winding 2 during the energizing or lift operation. A reverse current is thus caused to flow during the subsiding of self-induced voltages. The circuit provided extends from the power line 8 and switch Il) through leads II, I5 and I9, contacts 2U and 21, lead 28, tapped resistor 29, leads 40, 38, 49 and 4, to the magnet winding 2, and thence through leads 3, 25, 24, 48 and 49, resistor 41, lead 5|), contact 36, leads 35, 33, 38 and I2 to switch Il) and power line 9.

Values may be chosen for the resistor 41 and the potentiometer 39 such that by proper adjustment of the slider of the potentiometer 39 there will be no current flow through the responsive arm of the bridge after transients have subsided and upon a constant voltage being impressed across the power terminals of the bridge. The current flow through the bridge arms will be dependent upon the respective resistance values and by the use of well known computation the ratios of resistances may be set to achieve this balanced condition. This circuit condition may be referred to as a steady state condition of balance and within limits the value of constant impressed voltage may be altered without causing current flow in the responsive arm when the bridge is so adjusted. However, with the bridge so adjusted, upon breaking the lift or energizing circuit for the winding 2 the large self-induced voltages upset the bridge to initially activate the contactor 2i as hereinbefore noted and to cause an unbalance of the bridge when reverse polarity potential is applied to the bridge through the contactor 2|.

As the transient voltage of self-induction decreases the voltage across the Winding 2 approaches the value of that across the lower portion of the potentiometer 39 and at the instant of equal potentials across these two bridge arms the current i'iow in the respoonsive arm will cease. In use this momentary or instantaneous balance occurs at a position of adjustment of potentioms eter 39 which does not necessarily coincide with the position of adjustment for steady state balance which may only be achieved in the absence of the transient discharge potential. Adjustment; is, however, so chosen that instantaneous balance is approached only after some reverse current is established in winding 2 and at this point the contactor 2| will relax and the deenergizing reverse potential will then be removed from the bridge, the circuit again being readied for a lift operation. Both the drop out voltage and time lag of the contactor 2| will aiect the response of the circuit and adjustment may be made accordingly.

The magnitude of reverse current at the time of drop out of the contactor 2| is inuenced by the reverse voltage impressed on the bridge circuit. The greater the impressed reverse Voltage the greater4 the demagnetizing eiect willbe. To

7` control the magnitude of this current thetapped resistor 2-9 is provided.' Thus, for` any given set,- ting ofv the potentiometer 39l a change in value of the resistor 29 will effect a change in magnitude of reverse current at themoment of in.- stantaneous balance. Thus'the operator is` pro.-r vided with means for adjustment tomeet'difierences in retentivity of various loadsbeing handled.

Additional control of the circuit may be had; through movement of the slider of the poten-.- tiometer 39. The setting of potentiometer 39Minfluences the promptness with which instantaneous balance is arrived at for anyw given setting of the resistor 29. Withv a given load, as the sslider is moved upwardthe instantaneous balance will occur sooner but the magnitude of reverse current will be smaller. Likewise, a down Ward movement of theslider of potentiometer .39 will prolong the duration off application of demagnetizingv current and increase rtlie magni-A tudev of the same. Thus, if the operator islhandling a low retentivity low reluctance load, a more prompt and certainv release of the same may be obtained if desired. The-range of= adjustability thus provided is such as tov make the control circuit of this invention adaptable to various magnets and the varyingv conditions of installation which may be encountered without need for special engineering foreach separate installation.

if it is desired that either manual or auto matic control of the demagnetizing operation be made available, the circuit of this invention may be modified as shownvin Fig.l 3; In this form of the circuit an electromagnet 8| having a winding 32, similar to the magnet l` of Figs. 1 4and 2, forms part-ofl an arnr ofr a` bridge circuit that is also similar toV that of Figs; l and 2. A resistor 8S and'a potentiometerSd-iform the other three armsv of'the bridge circuit and a rectifier 85anda relay coil Bti-of a relay'B-l form the responsive; arm ofthe bridge. The relay 81 has a setV offnormallyopen contacts-88 inv circuit with a contacter coil 89 of'a'contactor 90 which has normally open contacts 9| and 92 and normally closed cont-acts 93; Between one corner oj the bridge circuit and thenormally open contacts 92'is a variable resistor 94:

A manually operable switch 95is employed-as shown having three positions of its vselector arm designated as lift, off andA drop;' Joined to the lift contact ofthe switch 95is-a coil 9E, o a` contacter 91 that has normally" open contacts 98 and 99 and normally closed contacts 109. A discharge resistor lll'l is in` shunt circuit relation .with nthe magnet winding. 82 through the normally closed contacts i90, similarly asthe resistors di and 35 in Figs. 1 and`2`are inVv shunt with the magnet winding '2;

The operation of the circuitshown in Fig;. 3 is initiated by placing the selector arm of'the switch 95in lit position to energize coil 96 of the contactor 91. Contacts 98` and.89; close to place themagnet winding 82 across the power sourceand then contacts H39 opentoremovethe resistor le! 1 from shunt with the; winding.. 82.

To automatically demagnetize. the magnet.. 8i the, selector arm of the manual switch 95.1.is placedin .oir position. Contactor 91 is then deenergized and contacts |09 close slightly; before contacts .98;.and 99sopen to remove the V windingzifromtheiine.. Similarly as in;.the fcirf cuit cfu-Figs.: 1 and-:.2:the Lselifinducedpvoltages of 'i the winding f 82 r-.cause a currentflow:` in,;the responsive .farmi of the. bridge; circuitv andzthe relay 8:1 is activated: tofclose, thecontaets-Sa Cl sure of contacts 88 causes the coil 89.to. bey placed )across the-powersource and: to be entre gized sor as. tof close contacts 9il vand 92.; ClosJ ingncontacts 9k and 92` places the'bridge circuit across' the power supply' to` apply a :reverse de'- magnetizing` current' to the winding 82.l Thel automatic. operation. of the` bridge issimilar. tothatdescribed .in reference to Figs; 1 and 2.

f Upon; anl instantaneous. balance prevailing in the bridge circuit relay 8'1frela'xes. and the' circuit` is ready forthe next lift operation...

Iffifon'anyv reason: the operator desiresfmanual control of the demagnetizationhe may placefthe selector'. arm of'. the switch 95findrop position' instead. of: oi position. The effect of the-relay. 8.1-isf thus circumvented to: the extent that thaenergizationotcontactor may be manually# prolonged ias long;y as tdesired by hold. ing theswitchf 9B in the dropi position.

Fig.: 4 depicts; acont-roller circuit inl accordanc@ this-.invention in which loutonefoon-- tactor is utilized and in which the lift contactors.l of Rigs. 1,2.: and 3 are replaced vby a manual zswitch. The single contactorv |02: has normally; open; contacts |031- and. |84- and. corresponds to the contactor' 2-lof' Figs. 1 andy 2 andjthe' contacton9 of Fig. 3;. Goil- |05 ofthe contacter' |02 formsspart ofy theresponsive arm off. ai bridge: circuiti along.; with va rectifier |06. This bridge circuit isilike that .ofi therothers heretoforedescribed, having a magnet winding. 101:01? amagnetfl forming part- -of-. one-bridge arm and ayresistor: i09 and a-.potentiometerz H0 for-ming theother arms. A-Ldischarge resistor H yl in shunt relationship with magnet winding HHy andV a reverse;` current control resistor" H2V areY con.- nected aswshown. andare. similar to.those off the preceding circuits.

A manual-switch.- H3 :with normally-open contacts-414'; H5'A andnormally; closed: contact H6 is; ccnnectedso .as to Vcontrol admission .off power andftherclosure of thedischargeicircuit. Moving the'switchcontacts upward closesn contacts H4 andllinandopens. contacts llsftoiconnect the magnetwindingwl D1 'across thepowerrsourceand torsremove thef discharge resistor -H lfrom shunt relation-Withi-the winding. |01:`

For release of thefload theswitch'l I3 is placed in the position shown, the 'ensuing deenergization and'demagnetization of themagnet llllA` is entirely automatic and` proceeds in a manner similar to that described for' crcuitsof 1; 2 and 3.' The resistor IH provides ashuntpath to carry the currents caused by self-induction and acts to limit peak voltage values. The bridge action and its effect 'upon the contactor' I 02'is the same as that discussed in connectionwith Figs. 1,2 and 3.

I claim;

1. In a. control.circuit foren inductive electromagnet a bridge network comprising. a. pair of terminals for connection to' the.coil` of, the inductive electromagnet to becontrolled to form a first bridge arm', a set of three* circuit impedances joined to one another and'A said first `bridge arm serially to form a four armed vbridgeloop with junctions interposed between the arms to prov-idel a pair'oi diagonally opposite input junet1ons and a pair ofdiagonally opposite reference junctions,A and a responsivebridge.arm including an unidirectional current .valve andi-ai control relavmeans havinga coil connected;.seria11yzbe.. tween Sadpair of diagonallyoppositelreference junctions, Sadrelay meanshvingcoiliconxrglled normally open contacts; an energizing circuit comprising switch means and currententry and current exit leads joining said pair of magnet terminals through said switch means when closed to a direct current source of predetermined polarity, said leads being oriented with respect to the polarity of said source and said current valve to render the latter non-conductive in relation to the potential imposed through said switch means by said source and conductive with respect to self-induced `potential generated in said first bridge arm upon opening of said switch means; and a deenergizing circuit including said control Vrelay means contacts joining the input junctions of said bridge network to said direct current potential source to apply a potential of reversed polarity to said first bridge arm when said relay means contacts are closed by excitation resulting from self-induced voltage in said first bridge arm.

2. In a vcontrol circuit for an inductive electromagnet a bridge network comprising a pair of terminals for connection to the coil of the inductive electromagnet to be controlled to form a first bridge arm, a set of three circuit impedances joined to one another and saidiirst bridge arm serially to form a four armed Ibridge loop with junctions interposed between the arms to provide a pair of diagonally opposite input junctions and a pair of diagonally opposite reference junctions, and a responsive bridge arm comprising a unidirectional current valve and a control relay means having a coil connected serially between said pair of diagonally opposite reference junctions, said relay means having coil controlled normally open contacts; energizing circuit switch means having two sets of contacts one set of which is closed when the other is open; an energizing circuit comprising a set of said switch means contacts and current entry and current exit leads joining said pair of magnet terminals through said set of contacts when closed to a direct current source of predetermined polarity, Said leads being oriented with respect to the polarity of said source and said current valve to render the latter non-conductive in relation to the potential imposed through said set of contacts by said source and conductive with respect to self-induced potential generated in said first bridge arm upon opening of said set of contacts; a discharge impedance; a discharge circuit comprising said discharge impedance and the second set of said switch means contacts joined serially between said set of magnet terminals; and a deenergizing circuit including said control relay means contacts joining the input junctions of said bridge network to said direct current potential source to apply a potential of reversed polarity to said rst bridge arm when said relayy means contacts are closed by excitation resulting from self-induced voltage in said first bridge arm.

3. In a lcontrol circuit for an inductive electromagnet a bridge network comprising a pair of terminals for connection to the coil of the inductive electromagnet to be controlled to form a first bridge arm, a resistance and a potentiometer having a movable mid-tap joined to kone another and said first bridge arm Yserially to form a four armed bridge loop with junctions interposed between the arms which junctions comprise a pair of diagonally opposite input junctions composed of the junctions between the rst bridge arm and the potentiometer and between the resistance and the potentiometer and a pair rof diagonally opposite reference `junctions comr`10 posed of the potentiometer mid-tap and the junction between the said rst bridge arm and said resistance, and a responsive bridge arm comprising a unidirectional current valve and a control relay means having a coil connected'serially between said pair of diagonally opposite reference junctions, said relay means having coil controlled normally open contacts; an energizing circuit comprising switch means and current entry and current exit leads joining said pair of magnet terminals through said switch means when closed to a direct current source of predetermined polarity, said leads being oriented with respect to the polarity of said source and said current valve to render the latter non-conductive in relation to the lpotential imposed through said switch means by said source and conductive with respect to self-induced potential generated in said first bridge arm upon opening of said switch means; .and a deenergizing circuit including said control relay means contacts joining the input Ajunctions of said bridge network to said direct current potential source to apply a potential of reversed polarity to said first bridge arm when said relay means contacts are closed by excitation, resulting from self induced voltage in said rst bridge arm and to cause demagnetization current of increasing value to now in said rst bridge arm whereby instantaneous balance of said bridge network is had for a predetermined value of rst bridge arm current in accordance with the setting of the mid-tap for said bridge ,y network potentiometer to deenergize said control relay means coil.

4. In a control circuit for an inductive electromagnet a bridge network comprising a pair of terminals for connection to the coil of the inductive electro-magnet to be controlled to form a first bridge arm, a set` of threecircuit impedances joined to one another and said iirst bridge arm serially to form a four armed bridge loop with junctions interposed between the arms to provide a pair of diagonally opposite input junctions and a pair of diagonally opposite reference junctions, and a responsive bridge arm comprising a unidirectional current valve and a control relay having a coil connected serially between said pair of diagonally opposite reference junctions, said relay having coil controlled norm-ally f open contacts; an energizing circuit comprising switch means and current entry and current exit leads joining said pair of magnet terminals through said switch means when closed to a direct current source of predetermined polarity,

said leads being oriented with respect to the polarity of said source and said current valve to v render the latter non-conductive in relation to the potential imposed through said switch means bysaid sourcekand conductive with respect to self-induced potential generated in said nrst bridge arm upon opening of said switch means; a deenergizing relay having a coil and normally open contacts operated thereby; a deenergizing circuit including said deenergizing relay contacts joining the input junctions of said bridge network .to said direct current potential source to apply Va potential of reversed polarity to said rst bridge arm'when said deenergizing relay contacts are closed; a rst circuit for said deenergizing relay coil connecting said deenergizing relay coil to a potential through said control relay contacts when closed; and a second circuit for said deenergizing rel-ay coil including manual switch means joining said deenergizing relay coil to a potential through said switch means when closed.

11 Y5. In a control circuit for an inductive electromagnet a pair of terminals for connection to the coil of the inductive electro-magnet to be controlled; an energizing circuit cimprising switch means and current entry and current exit leads joining said pair of magnet terminals through said switch means when closed to a directcurrent source of predeterminedpolarity; a bridge network comprising a rst bridge arm including said pair of terminals, a set of three circuit `impedances joined to one another and said first bridge arm serially to form a four armed bridge loop with junctions interposed between the arms to a pair of diagonally opposite input junctions and a pair of diagonally opposite reference junctions, and a responsive bridge arm including control relay means having a coil connected between said pair of diagonally opposite reference junctions, said relay means having coil controlled normally open contacts; circuit means in saidbridge net work acting to preclude energization of said coil of said control relay means upon closure of said energization circuit and rendering said relay means coil conductive with respect to self-induced potential generated in said rst bridge arm upon opening of said switch means; and a deenergizing circuit including said control relay means contacts joining the input junctions of said bridge network to said direct current potential source and oriented with respect to said current entry and current exit leads of said energizing circuit to apply a potential of reversed polarity to said nrst bridge arm when said relay means contacts are closed by excitation resulting from self-induced voltage in said rst bridge arm.

6. In a control circuit for an inductive electromagnet a pair of terminals for connection to the coil of the inductive electro-magnet to be controlled; energizing circuit switch means having two sets of contacts one set of which is closed when the other is open; an energizing circuit comprising one set of said switch means contacts and current entry and current exit leads joining said pair of magnet terminals through said pair of switch means contacts when closed to a direct current source of predetermined polarity; a bridge network comprising a rst bridge arm including said pair of terminals, a set of three circuit impedances .joined to one another and said first bridge arm serially to form a four armed bridge loop with junctions interposed between the arms to provide a pair of diagonally opposite input junctions and a pair of diagonally opposite reference junctions, and a responsive bridge arm including control relay means having a coil connected between said pair of diagonally opposite reference junctions, said relay means having coil controlled normallyiopen contacts; circuit means in said bridge network acting to preclude energization of said coil of said control relay means upon closure of said energizing circuit and rendering said relay means coil conductive with respect to self-induced potential generated in said first bridge arm upon opening of said set of contacts in said energizing circuit; a discharge impedance; a discharge circuit comprising said discharge impedance and the second set of said switch means contacts Ajoined serially between said set of magnet terminals; and a deenergizing circuit including said control relair means contacts joining the input junctions of said bridge network to said direct current potential source and oriented with respect to 4said current entry and current exit leads. Qi Sad ener- 'gizing circuit'to apply a potential of reversed said pair of terminals, a set of three circuit irnr'pedances joined to one another and said rst bridge arm serially to form a four armed bridge loop with junctions interposed between the arms to provide a pair of diagonally opposite input junctions and a pair of diagonally opposite reference junctions, and a responsive bridge arm having circuit means operable to preclude current flow in the bridge arm and a controlrelay means having a coil connected serially between said pair of diagonally opposite reference junctions, said relay means having coil controlled normally open contacts; said circuit means being oriented with respect to said energizing circuit to preclude energization of said relay means coil upon closure of said energizing circuit and to render said coil conductive with respect to selfinduced potential generated in said first bridge arm upon opening of said switch means; and a deenergizing circuit including said control relay means contacts joining the input junctions of said bridge network to said direct current potential source and oriented with respect to said current entry and current exit leads of said energizing circuit to apply a potential of reversed polarity. to'said first bridge arm when said relay means contacts-are closed vby excitation resulting from self -induced voltage in said iirst bridge arin.

8. In a control circuit for an inductive electro- .magnet a pair of terminals for connection to the coil of the inductive electro-magnet to be con- Atrolled; an energizing circuit comprising switch means and current entry and current exit leads joining said pair of magnet terminals through said switch means when closed to a direct current source of predetermined polarity; a bridge network comprising a first bridge arm including said pair of terminals, a set vof three circuit im- "pedances joined to one another and said first bridge arm serially to form a four armed bridge -loop with junctions interposed between the arms to provide a pair of diagonally opposite vinput junctions and a pair of diagonally opposite reference junctions, and a responsive bridge arm including contact closing means having an operating coil connected between said pair of diagonally opposite reference junctions, said contact closing means having normally open coil controlled contacts and being oriented with respect to said energizing circuit to preclude energization of said coil toractuate said contacts upon closure of said energizing circuit and to render said coil conductive with respect to self-induced potential generated in said rst bridge arm for contact .closure upon opening of said switch means; and a .deenergizing circuit including said contacts joining the input junctions of said bridge network to said direct current potential source and oriented vwith respect to said current entry and current exit leads of said energizing circuit to apply a potential of reversed polarity to said rst bridge arm when Said contacts are closed by excitation resulting from self-induced voltage in said first bridge arm.

9. In a control circuit for an inductive electromagnet a pair of magnet terminals for connection to the coil of the inductive electro-magnet to be controlled; a bridge network comprising a rst bridge arm including said magnet terminals joined between a first input junction and a first responsive junction, a set of three resistances joined serially between said first input and responsive junctions to form a four armed bridge loop with said first bridge arm, said resistances being joined to one another at junctions diagonally opposite said first input and responsive junctions to form a pair of diagonally opposite input junctions and a pair of diagonally opposite responsive junctions, and a responsive bridge arm including a relay control means coil joined between said responsive bridge junctions, said coil having coil controlled normally open contacts; an energizing circuit comprising switch means and connecting leads joining said magnet terminals to a direct current potential source through said switch means when closed; and a deenergizing circuit comprising said relay control means contacts and connecting leads joining said bridge network at said input junctions to the direct current potential source through said relay control means contacts when closed.

WILLIAM JOHN HUDSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 967,186 Hall Aug. 16, 1910 1,276,589 Steinmetz Aug. '210, 1918 1,923,311 Hodgson Aug. 22, 1933 2,020,671) Wright Nov. 12, 1935 2,522,259 Fay Sept. 12, 1950 

